1. Field of the Invention
The present invention relates to ionomer/high density polyethylene (HDPE) blends with improved flow and a method of achieving the same. More specifically but not by way of limitation, the present invention relates to the addition of low molecular weight ethylene/acrylic (methacrylic) acid copolymer to an ionomer/high density polyethylene blend in order to reduce viscosity of such blend and improve flow characteristics without significantly degrading physical properties such as impact resistance and tensile strength.
2. Description of the Related Art
It is generally known in the art to employ a thermoplastic alloy based on a blend of ionomer and high density polyethylene for injection molding exterior automotive molded-in-color fascia, bumper covers, side moldings and other decorative trim. These types of polymer blends are disclosed, for example, in U.S. Pat. No. 4,387,188 and are commercially available under the tradename Bexloy(copyright) from E. I. du Pont de Nemours and Company. They involve a copolymer of an xcex1-olefin, typically ethylene, copolymerized with an xcex1,xcex2-ethenically unsaturated carboxylic acid, typically acrylic acid, methacrylic acid or mixtures thereof, wherein the acid copolymer has preferably from 5 to 80 percent of the acid groups neutralized with metal ions such as zinc, sodium or the like. Such ionomers are commercially available under the tradename Surlyn(copyright) from E. I. du Pont de Nemours and Company. In the ""188 patent, the partially neutralized acid copolymer is blended with a linear polymer of xcex1-olefins and glass fiber to produce a molding resin. One difficulty with such blends is the achieving and sustaining optimum rheology without compromising physical properties such as impact resistance and tensile strength.
In view of the above mentioned problem, it has now been discovered that the addition or incorporation of a low molecular weight copolymer of ethylene copolymerized with acrylic acid, methacrylic acid and mixtures thereof improves the flow properties of the resulting blend without significantly degrading physical properties such as impact resistance and tensile strength. It has further been discovered that the incorporation of a maleic anhydride grafted high density polyethylene (MAN-g-HDPE) into such a flow modified ionomer/high density polyethylene blend for purposes of improving impact resistance does not significantly compromise the flow improvement.
Thus, the present invention provides an ionomer/high density polyethylene blend having improved flow properties comprising for every one hundred parts by weight of ionomer/high density polyethylene blend from one to about twenty parts by weight low molecular weight copolymer of ethylene and at least one other comonomer selected from the group consisting of acrylic acid, methacrylic acid and mixtures thereof, wherein said low molecular weight copolymer has a melt index (ASTM D1238) as low as 350 but more typically greater than 900 dg/min and an acrylic acid and methacrylic acid combined comonomer content of at least 5 weight percent.
The present invention further provides a method of reducing the viscosity of an ionomer/high density polyethylene blend without significantly degrading impact resistance and tensile strength comprising the step of blending for every one hundred parts by weight of ionomer/high density polyethylene cumulatively in the final blend from one to about twenty parts by weight low molecular weight copolymer of ethylene and at least one other comonomer selected from the group consisting of acrylic acid, methacrylic acid and mixtures there of, wherein said low molecular weight copolymer has a melt index (ASTM D1238) as low as 350 but more typically greater than 900 dg/min and an acrylic acid and methacrylic acid combined comonomer content of at least 5 weight percent.
Preferably the low molecular weight copolymer has a melt index of at least 5,000 dg/min and an acrylic acid and methacrylic acid combined comonomer content of at least 9 weight percent. The flow modified ionomer/high density polyethylene blend may further comprise for every one hundred parts by weight of ionomer/high density polyethylene blend from one to about twenty parts by weight polyethylene modified with from 0.2 to 5.0 weight percent maleic anhydride comonomer.